In sheet processing machines, for example sheet printing machines, the sheets (i.e., printing media) to be printed are individually taken off the top of a stack of sheets as they are delivered, typically in an underlapping relationship, to a feed table. Suction-based lifting devices are then typically employed to individually remove the sheets from the stack. In order for these lifting devices to reliably engage the sheets, it is a necessary requirement that the vertical position of the upper surface of the top sheet must be maintained within a relatively narrow range of heights.
To keep the uppermost sheet within the required range, the sheets are typically stacked upon a pallet which can be raised as needed to compensate for the decline in the stack height as sheets are removed. Ordinarily, a motorized stack lift drive is provided for raising the pallet and stack of sheets so that the top surface of the uppermost sheet is within the proper height range. Since the speed that sheets are fed to the printing machine (i.e., processing machine) depends on the operating speed of the printing machine, and the height of the top of the stack varies in dependence on the sheet thickness, the drive must raise the pallet in dependence on these parameters.
To ensure that the stack is raised to the proper height, generally a stack height sensor is utilized. A control device monitors output signals from the stack height sensor, the control device operating the drive in accordance with the signals. Either discontinuous (i.e. intermittent) or continuous pallet lifting can be performed to maintain the proper stack height as the sheets are removed from the stack.
German patent DE 3 607 979 A1 discloses one such control device that continuously raises a stack lift drive. In this system the stack lift drive motor is continually operated, while the lifting motor is sped up or slowed down as needed to adjust for the varying deviations between a predetermined desired height and the actual height as determined by the stack height sensor. Although this types of system can handle the stack lifting task, a disadvantage of this system is that the motor and accompanying drive must be capable of a very large range of adjustments to ensure that the top of the stack can be kept within the narrow height range at all processing speeds of the printing machine and for all printing medium thicknesses. As can be readily appreciated, such a system is relatively complex and expensive due to the extensive performance requirements for the motor and drive.
Drives and motors that operate discontinuously to lift the stacks have lower performance requirements. One such system is disclosed in British patent 1,535,474 (corresponding to German Patent No. DE 2 659 511 C2). Instead of varying the motor speed, to operate a discontinuous drive the drive is either switched on or off to adjust the stack height in accordance with the output signal of a stack height sensor. With these type of systems, however, the higher the printing speed and/or the thicker the printing media, the higher the switching frequency must be in order to keep the top of the stack at the required height. Accordingly, the drive and motor must be designed to handle such rapid intermittent operation. More significantly, at high switching frequencies the pallet and stack tend to vibrate, particularly in the associated mechanical lifting gears. These vibrations cause the stack (including the critically positioned top sheet) to experience unpredictable movements. As a result of these vibrations, when dealing with either height speed printing or thick printing media that has a relatively heavy weight, problems often occur in attempting to reliably lift the top sheet off of the stack and accurately transport the sheets to the feed table.
Another system for controlling the lifting of the sheets is disclosed in U.S. Pat. No. 4,832,329 (corresponding to German Patent No. DE 3 631 456 C2). In this system a correction movement is calculated and performed based on a number of complex factors. A pulse generator connected to the shaft of the motor counts fractions of motor shaft revolutions so that the exact amount of correction movement itself is stored in memory and factored into the next correction movement calculation. This exact measurement allows precise compensation for parameters such as temperature, system wear and tear, voltage irregularities, and so on. The speed of the processing machine is also known, but is only used to calculate precise sheet thickness and the number of sheets removed during a correction movement. Continuous correction movement is also discussed, however the system does not switch to a continuous lifting mode based directly on the speed of the printing machine and sheet thickness, but instead on a number of complex factors including time intervals between movements and precisely calculated sheet thicknesses. While again this system accomplishes its objective, this pulse generator and memory combination (means for determining a quantity dependent on correction movement) requires complex calculations and measuring devices when only a simple vertical movement is required. As a result, such a system is far more expensive, complex and error-prone than is necessary to keep a stack of sheets within the required height range.